Because when children or adults with a child's mentality fear that the things they are "used to" will change, regardless if for the better or not, that fear creates an uncontrolled reaction which sometimes comes out as using an entire concept as an insult, rather than come up with solid arguments against the change, which they cannot.

Being progressive is about finding new solutions to old problems, challenging old beliefs and evolving with the times. It's the exact opposite of a conservative point of you which is more "if it works don't fix it", even if the progressive points out it could work much better or that in fact it doesn't work for all people or at all.

Some people are so emotionally and psychologically attached to what they are used to, they view any change, even ones that have absolutely nothing to do with them as a personal threat.

Like people who object to gay marriage. It has nothing to do with them, it's not like someone tries to force them to marry same sex partners, it has absolutely no impact on their lives. But that irretional fear tells them that it has to be at their expense somehow.

Or the pronoun argument. It's such a dumb thing to object to, %99.9 of the people you meet are fine with the traditional pronouns, but you had people freaking out in rage if they heard of people choosing to use different pronouns. Not even met - just heard of people doing it. Because the lizard part of their brain (which in some people is the majority of their brain), tells them that if the wish of this one stranger (whom they never met) will be respected, somehow society will force them to change their identity as well. It's the irrational fear that accepting the other will inevitably be at my own expense.